Because retooling is expensive and time consuming. And there is no guarantee how long this level of demand will sustain. And no guarantees on cheap chinese goods flooding the market and taking all your customers in a few months.
There is some price per mask at which the N95 respirator manufacturers that currently exist in the US would be willing to hire more people, buy more supplies, etc and start running 24/7. The only reason they aren't already is because nobody has agreed to pay for it.
You might could fix that with strong tariffs. There was a guy recently, can’t remember his name, who was very much in favor of domestic manufacturing and tariffs against China. Seemed to catch a lot of flak for those policies! Wish I could remember who that was...